The Norwegian Fødselsnummer (Birthnumber) is an 11-digit number with 2 control digits. The 10-th digit is a control digit calculated with a weighted modulo 11 variant over the first 9 digits. The 11-th digit is a control digit calculated with another weighted modulo 11 variant over the first 9 digits combined with the 10-th control digit.
As in other countries also this number is based on the date of birth. The first 6 digits represent the birth date as “ddmmyy”. Problem with a 6-digit date is that you cannot identify the century – is a Fødselsnummer starting with 121009 someone born in 1909 or 2009? The Norwegian government has solved this by grouping the following 3 individual digits (individual number) in groups representing a certain era. If you are born between 1854-1899, then your individual number must be between 500 and 749, born between 1900-1999 then your number lies between 000 and 499, and for those born recently between 2000-2039 then your number lies between 500 and 999. With some exceptions for those with an individual number between 900 and 999.
Like in other countries the odd individual numbers are given for males, the even for females.
Be aware that by validating national personal identification numbers, like the Fødselsnummer, that contain a date part you cannot rely only on the control digits. The Fødselsnummer “31046812355” is completely valid if we look to the 10th and 11th control digit – however the birth date April 31 in 1968 did never occur!